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United States Patent |
5,708,556
|
van Os
,   et al.
|
January 13, 1998
|
Electrostatic chuck assembly
Abstract
An electrostatic support system for retaining a wafer. The support system
generally includes a support body having a support surface for retaining
said wafer, a voltage source coupled to the support body for
electrostatically coupling the wafer to the support surface, and a cooling
system for cooling the wafer. A plurality of arm members extend from the
support body to a carriage assembly for releasably mounting the support
body to the processing chamber with the support body and the arm members
separated from the chamber floor. This invention also includes the method
of supporting a wafer in a processing chamber which includes the steps of
positioning the wafer on a wafer supporting surface, applying a voltage to
an electrode assembly to electrostatically attract the wafer to the
support surface and, after processing the wafer, substantially grounding
the electrode assembly to sufficiently deactivate the electrostatic charge
for release of the wafer from the support surface.
Inventors:
|
van Os; Ron (Sunnyvale, CA);
Ross; Eric D. (Santa Cruz, CA)
|
Assignee:
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Watkins Johnson Company (Palo Alto, CA)
|
Appl. No.:
|
500480 |
Filed:
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July 10, 1995 |
Current U.S. Class: |
361/234 |
Intern'l Class: |
H02N 013/00 |
Field of Search: |
361/234
279/128
|
References Cited
U.S. Patent Documents
3634740 | Jan., 1972 | Stevko | 361/234.
|
3916270 | Oct., 1975 | Wachtler et al. | 361/234.
|
4184188 | Jan., 1980 | Briglia | 361/234.
|
4282267 | Aug., 1981 | Kuyel | 427/38.
|
4292153 | Sep., 1981 | Kudo et al. | 204/164.
|
4313783 | Feb., 1982 | Davies et al. | 156/643.
|
4324611 | Apr., 1982 | Vogel et al. | 156/643.
|
4384918 | May., 1983 | Abe | 156/643.
|
4399016 | Aug., 1983 | Tsukada et al. | 204/192.
|
4431473 | Feb., 1984 | Okano et al. | 156/345.
|
4512391 | Apr., 1985 | Harra | 165/48.
|
4514636 | Apr., 1985 | King | 250/443.
|
4565601 | Jan., 1986 | Kakehi et al. | 156/643.
|
4680061 | Jul., 1987 | Lamont, Jr. | 148/1.
|
4705951 | Nov., 1987 | Layman et al. | 250/442.
|
4709655 | Dec., 1987 | Van Mastrigt | 118/719.
|
4724621 | Feb., 1988 | Hobson et al. | 34/218.
|
4968374 | Nov., 1990 | Tsukuda et al. | 156/345.
|
5099571 | Mar., 1992 | Logan et al. | 29/825.
|
5221450 | Jun., 1993 | Hattori et al. | 204/192.
|
5325261 | Jun., 1994 | Horwitz | 361/234.
|
5330610 | Jul., 1994 | Eres et al. | 117/86.
|
5382311 | Jan., 1995 | Ishikawa et al. | 156/345.
|
5452177 | Sep., 1995 | Frutiger et al. | 361/234.
|
5452510 | Sep., 1995 | Barnes et al. | 29/825.
|
5460684 | Oct., 1995 | Saeki et al. | 156/345.
|
5467249 | Nov., 1995 | Barnes et al. | 361/234.
|
5522131 | Jun., 1996 | Steger | 361/234.
|
5522937 | Jun., 1996 | Chew et al. | 118/728.
|
5525159 | Jun., 1996 | Hama et al. | 118/723.
|
5539609 | Jul., 1996 | Collins et al. | 361/234.
|
Foreign Patent Documents |
63-292625A | Nov., 1988 | JP.
| |
3-76112A | Apr., 1991 | JP.
| |
Other References
George A. Wardly, Electrostatic Wafer Chuck for Electron Beam
Microfabrication, Rev. Sci. Instrum., vol. 44, No. 10, Oct. 1973, pp.
1506-1509.
|
Primary Examiner: Fleming; Fritz
Attorney, Agent or Firm: Flehr Hohbach Test Albritton & Herbert LLP
Claims
What is claimed is:
1. An electrostatic support system for supporting a wafer comprising:
a support body positionable in a processing chamber and having a support
surface for retaining said wafer, said support body including protective
means for preventing catastrophic separation of said wafer from said
support surface,
a voltage source coupled to said support surface for electrostatically
coupling said wafer to said support surface, and
a cooling system for cooling said wafer, said cooling system including a
source of a gaseous substance, a plurality of gas distribution grooves
formed in said support surface and configured for uniformly distributing a
gaseous substance between said wafer and said support surface, and a
conduit connecting said gas source to said gas distribution grooves for
flow of said gaseous substance from said gas source to said gas
distribution grooves,
said conduit including an inner plenum coupled to said gas source and an
outer plenum coupled to said gas distribution grooves, and said protective
means including means for restricting the flow of said gaseous substance
between said inner plenum and said outer plenum.
2. The support system of claim 1 in which said gas distribution grooves
divide said support surface into a plurality of segments having a
substantially uniform surface area.
3. The support system of claim 2 in which said segments have a
substantially planar surface.
4. The support system of claim 2 in which said segments each have a surface
area of about 1.5 to 2.5 square inches.
5. The support system of claim 1 in which said gas distribution grooves
have a depth of about 0.015 inch and a width of about 0.062 inch.
6. The support system of claim 1 in which said gas distribution grooves
have a substantially curved cross-sectional shape.
7. The support system of claim 1 in which said flow restricting means
controls the flow of said gaseous substance between said inner plenum and
said outer plenum at a rate such that when a portion of said wafer becomes
separated from said support surface, the pressure of said gaseous
substance in said gas distribution rings and said inner plenum is
insufficient to disengage the remainder of said wafer from said support
surface.
8. An electrostatic support system for supporting a wafer comprising;
a support body positionable in a processing chamber and having a support
surface for retaining said wafer, said support surface having a plurality
of circumferentially spaced holes formed therethrough, said support body
including protective means for preventing catastrophic separation of said
wafer from said support surface;
a voltage source coupled to said support surface for electrostatically
coupling said wafer to said support surface;
a cooling system for cooling said wafer, said cooling system including a
source of a gaseous substance, a plurality of gas distribution grooves
formed in said support surface and configured for uniformly distributing a
gaseous substance between said wafer and said support surface, and a
conduit connecting said gas source to said gas distribution grooves for
flow of said gaseous substance from said gas source to said gas
distribution grooves, said conduit including a plurality of passageways
each coupled to one of said holes extending through said support surface;
a lifting mechanism for moving said wafer relative to said support surface,
said lifting mechanism including a plurality of vertically extending
lifting pins each extending through a different one of said holes formed
through said support surface, said lifting pins being movable relative to
said support surface between an extended position with said lifting pins
extending through said hole in said support surface for supporting said
wafer above said support surface and a retracted position with said
lifting pins retracted beneath said support surface;
said conduit including an inner plenum coupled to said gas source and an
outer plenum coupled to said gas distribution grooves, and said protective
means including means for restricting the flow of said gaseous substance
between said inner plenum and said outer plenum.
9. An electrostatic support system for supporting a wafer comprising:
a support body positionable in a processing chamber and having a support
surface for retaining said wafer, said support surface having a plurality
of circumferentially spaced holes formed therethrough,
a voltage source coupled to said support surface for electrostatically
coupling said wafer to said support surface,
a cooling system for cooling said wafer, said cooling system including a
source of a gaseous substance, a plurality of gas distribution grooves
formed in said support surface and configured for uniformly distributing a
gaseous substance between said wafer and said support surface, and a
conduit connecting said gas source to said gas distribution grooves for
flow of said gaseous substance from said gas source to said gas
distribution grooves, said conduit including a plurality of passageways
each coupled to one of said holes extending through said support surface,
each of said passageways is formed with at least one flow restriction for
restricting the flow of said gaseous substance between said gas
distribution grooves and said gas source, and
a lifting mechanism for moving said wafer relative to said support surface,
said lifting mechanism including a plurality of vertically extending
lifting pins each extending through a different one of said holes formed
through said support surface, said lifting pins being movable relative to
said support surface between an extended position with said lifting pins
extending through said hole in said support surface for supporting said
wafer above said support surface and a retracted position with said
lifting pins retracted beneath said support surface.
10. An electrostatic support system for retaining a wafer in a processing
chamber having a chamber wall and a bottom, said support system
comprising:
a body portion having a support surface for retaining said wafer,
a plurality of arm members extending outwardly from said body portion for
supporting said body portion in said processing chamber with said body
portion and said arm members spaced from the bottom of said chamber, and
means for applying a charge to said support surface for electrostatically
coupling said wafer to said support surface.
11. The support system of claim 10, and further comprising a carriage
assembly mountable to said chamber, said arm members being mounted to said
carriage assembly.
12. The support system of claim 10 in which said arm members are mountable
to said chamber wall of said processing chamber.
13. The support system of claim 12 in which said chamber wall includes a
plurality of side wall portions and in which said arm members are
positioned on said body portion such that when said support system is
installed in said processing chamber, said arm portions are mounted to one
of said side wall portions.
14. The support system of claim 10 in which said support system includes
two of said arm members.
15. The support system of claim 10 in which said means for applying a
charge includes a voltage source remote from said body portion and at
least one electrical connector coupled to said voltage source and
extending through a passageway formed in one of said arm members.
16. The support system of claim 10 in which said means for applying a
charge includes at least one electrode carded by said body portion and an
electrode cooling system for cooling said electrode, said electrode
cooling system including a liquid source remote from said body portion and
a conduit extending through a passageway formed in one of said arm
members.
17. The support system of claim 16 in which said means for applying a
charge includes at least one electrical connector extending through a
passageway formed in another of said arm members.
18. The support system of claim 10, and further comprising biasing means
for applying an RF bias to said body portion.
19. The support system of claim 18 in which said biasing means includes a
source remote from said body portion and at least one electrical connector
extending through a passageway in one of said arm members.
20. The support system of claim 10, and further comprising a wafer cooling
system for cooling said wafer.
21. The support system of claim 20 in which said wafer cooling system
includes a source of a gaseous substance and a conduit coupling said
gaseous substance source to said support surface, said conduit extending
through a passageway formed in one of said arm members.
22. The support system of claim 21 in which said wafer cooling system
includes a plurality of gas distribution grooves formed in said support
surface.
23. The support system of claim 21 in which said support body includes
protective means for preventing catastrophic separation of said wafer from
said support surface.
24. The support system of claim 23 in which said conduit includes an inner
plenum coupled to said gas source and an outer plenum coupled to said
support surface, and in which said protective means includes means for
restricting the flow of said gaseous substance between said inner plenum
and said outer plenum.
25. The support system of claim 21 in which said conduit includes at least
one hole extending through said support surface.
26. The support system of claim 21 in which said body portion includes at
least one lifting member movable relative to said support surface between
an extended position with said lifting member extending through said hole
in said support surface for supporting said wafer above said support
surface and a retracted position with said lifting member retracted
beneath said support surface.
Description
BRIEF DESCRIPTION OF THE INVENTION
This invention relates in general to a method to hold down a conductive
member onto a support surface without contacting the front or side
surfaces of the conductive member, more particularly, to an electrostatic
clamp for supporting a semiconductor wafer during processing.
BACKGROUND OF THE INVENTION
Various support systems have been employed to support a semiconductor wafer
during chemical vapor deposition, sputtering, etching, and other
processes. The support systems are often cooled in an attempt to maintain
a substantially constant wafer temperature. Retaining the wafer at a
constant temperature during processing is important for controlling the
chemical process, obtaining process uniformity, and preventing damage to
the integrated circuitry already formed on the wafer. The wafer is
generally secured to the support to retain the wafer in position during
processing and to improve heat transfer between the wafer and the support
surface.
One type of support system employs a perimeter clamping ring which extends
across the peripheral edge of the wafer to retain the wafer in place. The
portion of the wafer beneath the ring is damped tightly against the
support surface. The clamping ring limits the total area available for
circuit formation since the peripheral edge of the wafer is covered by the
ring. The first side contact may introduce impurities to the wafer.
Another type of support system electrostatically clamps the wafer to the
support surface. This is accomplished by applying a voltage to the support
and inducing an image charge on the wafer. The different potentials
attract the wafer to the support surface, tightly clamping the entire
wafer to the support. The entire surface area of the wafer now becomes
available for the formation of integrated circuits. In addition, there is
no surface contamination.
Electrostatic support systems are typically supported on the bottom of the
processing chamber. As a result, the system pump must be positioned at
another location, for example to the side of the chamber. This
configuration reduces uniformity of the flow of process gases throughout
the chamber and increases the footprint of the overall system. Removing
the wafer support system from the bottom of the chamber would allow the
pump to be axially aligned with the wafer, improving uniformity and
reducing the footprint of the processing system.
OBJECTS AND SUMMARY OF THE INVENTION
It is a primary object of this invention to provide an electrostatic
support assembly for supporting a wafer during processing.
It is a further object of this invention to provide an electrostatic
support system for supporting a wafer during processing at low pressures
which provides for the uniform flow of process gases around the wafer.
It is another object of the invention to provide an electrostatic support
assembly with a cooling system for efficiently maintaining a constant and
uniform wafer temperature during processing.
It is yet another object of the invention to provide an electrostatic
support assembly in which the wafer may be rapidly and efficiently removed
from the support assembly.
A more general object of the invention is to provide an electrostatic
support system which securely retains the wafer during processing while
minimizing the risk of damage to the components on the exposed surface of
the wafer from excessive heat, transient currents, and premature
separation of the wafer from the support.
Another general object of the present invention is to provide an
electrostatic support system which allows the entire surface of the wafer
to be uniformly exposed to the process gases and which may be efficiently
manufactured and operated.
In summary, this invention provides an electrostatic support assembly which
is particularly suitable for retaining a wafer during processing. The
support assembly generally includes a support body having a support
surface for retaining the wafer, a voltage source coupled to the support
body for electrostatically coupling the wafer to the support surface, and
a cooling system for cooling the wafer. The cooling system includes a
plurality of gas distribution grooves formed in the support surface which
facilitate the rapid distribution of a gaseous substance between the wafer
and the support surface. The cooling system includes a restriction
mechanism in the conduit between the gas source and the gas distribution
grooves to prevent catastrophic separation of the wafer from the support
surface in the event a portion of the wafer becomes separated from the
support surface. A plurality of arm members extending from the support
body are mountable to the processing chamber with the support body and the
arm members separated from the chamber bottom.
This invention also includes the method of supporting a wafer in a
processing chamber which includes the steps of positioning the wafer on a
wafer supporting surface of a support body having at least one electrode,
applying a voltage to the electrode to induce the electrostatically
attractive forces that hold the wafer to the support surface and, after
processing the wafer, substantially grounding the electrode to
sufficiently deactivate the electrostatic charge for release of the wafer
from the support surface. In a preferred form of the invention, the
support body includes two electrodes and the voltage applying step
includes applying a positive voltage to one of the electrodes and a
negative voltage to the other electrode. After the wafer is removed from
the chamber, the polarity of the electrodes is reversed for the next
wafer. In an embodiment of the invention employed in plasma-enhanced
processes, the method also includes the step of applying an RF bias to the
electrode or combination of electrodes.
Additional objects and features of the invention will be more readily
apparent from the following detailed description and appended claims when
taken in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an electrostatic support system constructed
in accordance with the invention, shown installed in a processing chamber.
FIG. 1A is an enlarged, partially broken away view of a reactor
incorporating the electrostatic support system of FIG. 1.
FIG. 2 is a top view, partially broken away, of the electrostatic support
system of FIG. 1, shown supporting a wafer.
FIG. 3 is a top view of the support surface of the electrostatic support
system of FIG. 1.
FIG. 4 is a cross-sectional view taken substantially along line 4--4 in
FIG. 3, shown with the lifting pins in an extended position supporting a
wafer.
FIG. 5 is a bottom plan view of the base of the electrode assembly.
FIG. 6 is an enlarged schematic cross-sectional view, partially broken
away, of the electrode assembly and DC and RF voltage supplies of the
invention.
FIG. 7 is an enlarged cross-sectional view, partially broken away, of the
ion focus ring and guard ring of the support body 12.
FIG. 8 is an enlarged cross-sectional view, partially broken away, of the
lifting mechanism, shown with the lifting pins in the retracted position
and a wafer positioned on the support surface.
FIG. 9 is cross-sectional view taken substantially along line 9--9 in FIG.
4.
FIG. 10 is a cross-sectional view taken substantially along line 10--10 in
FIG. 8.
FIG. 11 is a cross-sectional view taken substantially along line 11--11 of
FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
Reference is now made in detail to the preferred embodiment of the
invention, which is illustrated in the accompanying figures. Turning now
to the drawings, wherein like components are designated by like reference
numerals throughout the various figures, attention is directed to FIGS.
1-2.
FIGS. 1 and 2 generally show an electrostatic support system or chuck
assembly 10 which is particularly suitable for retaining a wafer 6 in a
processing chamber 8 during plasma-enhanced chemical vapor deposition,
although the support system 10 may also be used with other types of
processing including, but not limited to, chemical vapor deposition,
sputtering, etching and the like. The support system 10 is particularly
suitable for use with a high density, plasma enhanced chemical vapor
deposition system of the type shown for example in FIG. 1A and disclosed
in the co-pending application Ser. No. 08/500,493, incorporated herein by
reference. The support system 10 includes a support body 12 with a support
surface 14 for retaining a wafer and arm members 16 extending outwardly
from the support body 12 for mounting the body to the chamber. In the
present embodiment, arm members 16 are mounted to a carriage assembly 17,
which is in turn releasably secured to the chamber 8. In FIG. 1A, the
processing chamber 8 is positioned below a plasma assembly 9 and includes
a gas injection manifold 11 mounted to the chamber 8 for injecting gaseous
substances into the processing chamber. Below the chamber 8 is a vacuum
pump 13 coupled to the chamber 8 by a port 15 which defines the bottom of
the processing chamber. The support system 10 is suspended above the
bottom of the process chamber, allowing the pump 13 to be substantially
axially aligned with the process chamber. During operation, the entire
wafer 6 is securely clamped to the support surface 14 by an electrostatic
force. The uniform contact provided by the electrostatic clamping controls
the heat transfer between the wafer 6 and the support surface 14 so that
the entire wafer may be retained at the desired temperature throughout the
processing. Maintaining the wafer at a uniform temperature is required for
achieving the desired film properties for material deposition or uniform
etch rates for removal of material.
Support body 12 includes an electrode assembly 20, shown particularly in
FIGS. 4-7, which generally defines the support surface. The electrode
assembly 20 is mounted to support base 18 having a removable cover 19 to
provide access to the interior of support body 12 for maintenance and
repair. Suitable seal members are provided at each junction.
The electrode assembly 20 includes an inner electrode 22, an annular outer
electrode 24 and an annular guard ring 26 mounted to a base 28 which is
electrically isolated from the electrodes, but not from the guard ring.
The inner electrode 22, outer electrode 24 and guard ring 26 are coated
with a dielectric material, electrically isolating the electrodes 22 and
24 from each other as well as the processing chamber. The peripheral edges
of the electrodes 22 and 24 and guard ring 26 are preferably curved so
that the dielectric coating may be uniformly applied, providing protection
against arcing. In the present embodiment, the base 28 is formed of
aluminum and coated with a high quality dielectric coating such as
aluminum oxide. However, it will be understood that other means may be
used for electrically isolating base 28 from the electrodes and the guard
ring. A fluid channel 30 extends through the base and is enclosed by a
cover plate 32 mounted to the base 28. As is shown particularly in FIG. 5,
the channel 30 snakes across a substantial portion of the base 28 to
provide uniform cooling and to avoid other components of the support body
12. The inlet 31 and outlet 33 of the channel are coupled via conduits 35
(FIG. 2) to a fluid source 36 for the circulation of a cooling fluid such
as water or another suitable liquid through the channel. A gasket 34 is
compressed between the base 28 and cover plate 32 to form a seal between
the base and the cover plate and prevent leakage of the cooling fluid from
the base 28.
The channel 30 provides a passageway through the base 28 for a cooling
fluid such as water or another liquid through the base 28 for efficiently
removing heat from the electrode assembly 20. The electrode assembly 20 of
this invention is of particular advantage in that the electrodes 22 and 24
and guard ring 26 are electrically isolated from the grounding effects of
the cooling fluid. Transporting the cooling fluid through the base 28 is
preferred as heat may be rapidly removed, eliminating temperature
gradients and preserving the planarity of support surface 14. However, in
other modifications of the invention the electrode cooling system may be
separate from the base 28 and electrodes 22 and 24. Furthermore, it can be
used as a means to set the desired temperature of the assembly at a
process defined temperature.
Electrode system 20 is used to electrostatically attract the wafer 6 to the
support surface 14. As is shown particularly in FIG. 6, the electrode
system 20 includes a first electrical connector 40 coupling the inner
electrode 22 to a voltage source 42 through an RC filter 43 and a second
electrical connector 44 coupling the outer electrode 24 to the voltage
source 42 through an RC filter 45. The voltage source 42 applies a DC
bias, with the polarity of the outer electrode 24 being opposite that of
the inner electrode 22. The applied DC potentials on the electrodes 22 and
24 generate image charges on the back surface of the wafer and
electrostatically attract the wafer to the support surface 14, securely
clamping the wafer to the support surface. The polarity of the electrodes
is reversed by activating gang switch 46 each time a wafer is removed from
support surface 14 to minimize any residual charges or polarization in the
dielectric coating which may interfere with the release of the wafer from
the support surface.
The surface area of the inner and outer electrodes 22 and 24 is
substantially equal, providing a uniform bipolar charge across the support
system 10. When the support system 10 is used in plasma enhanced systems,
the electrodes 22 and 24 are electrically floating with respect to the
plasma to prevent leakage of current to the plasma. Electrodes 22 and 24
are preferably referenced by center tapping the electrodes to the guard
ring 26, which substantially centers the DC potential around the potential
induced in the wafer by the plasma. As a result, a constant potential is
distributed across the support surface to maintain uniform clamping. In
the present embodiment, where the support system 10 is used in
plasma-enhanced processing systems, the wafer has a potential of about
-300 V, for a typical RF bias of 2000 watts. Centertapping the electrodes
22 and 24 to the guard ring 26 causes one electrode to have potential of
+100 V and the other electrode a potential of -700 V when coupled to an
800 V voltage source. If the electrodes were centertapped to ground, the
potential of the electrodes would be non-uniform with respect to the
biased wafer. If the voltage source has a sufficiently high impedance so
that the power supply is fully floating from ground, center tapping the
voltage source to the guard ring 26 will be unnecessary because the
electrodes 22 and 24 will "self center" around the bias induced in the
wafer, accommodating the desired uniform clamping.
The electrodes 22 and 24 preferably have substantially smooth, planar upper
surfaces for improved temperature uniformity across the wafer to maintain
the planarity of support surface 14. The increased stability provided by
the planarity of the electrode surfaces is particularly important during
high-density plasma enhanced processes where the film deposition is
occurring at the high thermal loads of the wafer where thermal
non-uniformities can cause stress in the wafer leading to wafer breakage.
By preventing possible damage to the wafer, the electrode assembly
substantially improves the efficiency of the processing system.
The electrostatic charge between the wafer 6 and the support surface 14
must be deactivated before the wafer may be safely removed from the
support body 12. In a preferred embodiment, the electrodes 22 and 24 are
discharged by switching the electrodes from the voltage supply 42 to
ground 44 through resistors 49 having a resistance of about 100 k.OMEGA..
The resistors control the rate of discharge, preventing the development of
voltage transients which may damage the components on the wafer surface.
The application of any RF bias to the support system 10 is also
discontinued. During this time, the plasma source of the processing system
remains active causing any charge remaining in the wafer to drain. By
effectively grounding the electrodes 22 and 24 and the wafer 6, the
electrostatic field is removed and the wafer may be easily and safely
removed from the support surface 14.
In the present embodiment, the support system 10 is particularly adapted
for use with plasma-enhanced processing systems. Electrode assembly 20
includes means for applying an RF bias to the support body. As is shown
particularly in FIG. 6, electrode assembly 20 includes an electrical
connector 52 which couples the inner and outer electrodes 22 and 24,
respectively, to an RF source 54 through a matching network 56. The
self-induced bias created on the wafer by applying the RF bias to the
support surface accelerates ions from the plasma sheath toward the wafer.
These energized ions are required to prevent formation of voids during
step coverage chemical vapor deposition.
The frequency of the RF bias applied to the support surface 14 is within
the range of 450 kHz to 60 MHz. Preferably, the RF frequency of the plasma
source is different from that of the chuck to minimize frequency beating.
In the present embodiment, the frequency of the RF source 54 is
approximately 3.39 MHz for a plasma source frequency of approximately
13.56 MHz.
A preferred matching network 56 is shown schematically i FIG. 6. The
matching network 56 generally includes an inductor 57 and a pair of
variable capacitors 58 and 59 coupled to the inductor 57 for phase
matching the RF source 54 to the wafer 62 to change the RF delivery
pattern to the wafer. The support body 12 has a well defined, capacitive
inductance of about 4-7 .OMEGA. at 50.degree. to 75.degree. C. during the
process. The range of the adjustable capacitors 58 and 59 and the inductor
57 have been designed such that, if no plasma is present, the match is out
of range causing the RF generator to reduce its output level to prevent
damage to the support assembly 10 due to excessive voltage in the unloaded
resonant circuit. The inductor 57 has an iron powder core material for
minimizing inductance loss. The high permeability of the powder reduces
the number of turns which are required, improving the Q value and space
requirements of the inductor. For an RF source frequency of about 3.39
MHz, the inductor 57 preferably has an inductance of about 13 .mu.H.
Matching network 56 is preferred as it offers an increase in efficiency of
about fifty percent compared to standard systems. However, it is to be
understood that in other modifications of the invention other types of
matching networks may be employed.
In a preferred embodiment of the invention, the DC voltage source 42 and
the RF bias source 54 are combined as shown in FIG. 6. Both sources 42 and
54 are coupled directly into the electrodes 22 and 24, with the DC voltage
source 42 being tied to the same connection through an RC filters 43 and
45. Combining the DC source 42 and RF source 54 reduces the number of
connections to the support body, minimizing the overall size required for
the support body 12. This is of particular advantage with the present
invention, where raising the support body 12 above the bottom of the
chamber limited the amount of available space. Capacitors 62 positioned
between the RF source 54 and the electrodes 22 and 24 control the ratio of
the application of the RF bias to the inner electrode 22 and the outer
electrode 24, providing an adjustment mechanism for obtaining a uniform RF
distribution across the entire wafer for ensuring uniform processing on
the wafer surface.
As is set forth above, the guard ring 26 may be coupled to the centertap of
the DC voltage source 42 when support system 10 is used with
plasma-enhanced processing systems to provide a uniform DC voltage across
the wafer. In addition, the guard ring 26 may be tied to the RF source 54
to provide a means of further controlling the RF bias delivered to the
wafer plane.
The guard ring 26 may be formed of an electrically conductive material such
as aluminum. By tying the electrically conductive guard ring 26 to the RF
source as shown for example in FIG. 6, a direct measurement of the induced
wafer bias may be obtained at junction 64 when the electrical contact with
the wafer is established.
Support body 10 includes an ion focus ring 72 which extend around the
periphery of the electrode assembly 20 to protect the active electrodes
from the plasma. The ion focus ring 72 is formed of a ceramic material or
other suitable dielectric material. As is shown particularly in FIG. 7,
the wafer extends across the guard ring 26 and a portion of the ion focus
ring 72. In the present embodiment, the ion focus ring 72 includes an
annular shoulder 74 for supporting the wafer so that the ion focus ring 72
at least partially protects the peripheral edge of the wafer from the
plasma. Any plasma seeping between the wafer and the ion focus ring 72
will encounter the guard ring 26, which will effectively isolate the
plasma from the active electrodes 22 and 24.
Wafer 6 is lowered onto and raised from support surface 14 by a wafer
lifting assembly, generally designated 86. Lifting assembly 86 includes a
plurality of lifting pins 88 which extend through apertures 90 formed in
the support surface 14 and the electrode assembly 20. The lifting pins 88
are movable between the extended position shown in FIG. 4, with the pins
88 retaining the wafer 6 above the support surface 14, and the retracted
position shown in FIG. 8. In the present embodiment, lifting mechanism 86
includes three lifting pins 88 (FIG. 1), the minimum number of pins
required for evenly supporting wafer 6 as it is moved relative to the
support surface. Pins 88 are located approximately halfway between the
center and peripheral edge of the wafer. Although using a minimum number
of pins is preferred, it is to be understood that the number and position
of pins 88 and apertures 90 may be varied as desired.
In the present embodiment, the three lifting pins 88 are carried by a yoke
member 92. The yoke member 92 is moved back and forth relative to the
support surface 14, moving the lifting pins 88 between the extended and
retracted positions shown in FIGS. 4 and 8, by an actuator 94 disposed
between the electrode assembly 20 and yoke member 92. Movement of the
lifting pins 88 is synchronized with the yoke member 92, ensuring the
wafer 6 is retained in a substantially horizontal orientation as it is
moved by the lifting pins 88. The actuator 94 of the present embodiment is
provided by a pneumatic cylinder, although it will be understood that
other actuating means may also be used to raise and lower the yoke member
92.
As is shown particularly in FIG. 8, lifting pin 88 is mounted to a collar
96. In the present embodiment, collar 96 is preferably formed of a plastic
material for increased compliance and reduced friction as the pin 88 is
moved between the retracted and extended positions. The lifting pin 88 and
collar 96 extend through the longitudinal bore of a bearing 98. In the
present embodiment, the bearing 98 is positioned in an opening 99 formed
in the base 28 of the electrode assembly 20. A shaft 100 couples the
collar 96 to a socket 102 carried by the yoke member 92. The collar 96,
shaft 100 and socket 102 and surrounded by a bellows assembly 104 which is
mounted to the electrode assembly 20 and the yoke member 92. Suitable seal
rings are compressed between bellows assembly 104 and the electrode
assembly and yoke member. The bellows assembly 104 expands and contracts
and the yoke member 92 is raised and lowered relative to the electrode
assembly 20. As is described in more detail below, the lifting mechanism
86 of the present invention is of particular advantage in that it
cooperates with the wafer cooling system in the delivery of a gaseous
substance to the support surface 14. It will be understood that the
various components of the lifting mechanism 86 may be varied within the
scope of the present invention. Moreover, it will be understood that in
other modifications of the invention, support system 10 may employ
different mechanisms for removing the wafer 6 from the support surface.
Support system 10 employs a gaseous cooling system for cooling the wafer
during processing. A non-reactive gaseous substance, such as helium,
argon, oxygen, hydrogen and the like, is distributed between the support
surface 14 and the wafer 6 to provide substantially uniform cooling across
the entire wafer. Maintaining the entire wafer at a uniform temperature
during processing significantly improves the uniformity of the layers
formed on the wafer surface.
In the present embodiment, the gaseous substance is delivered through the
lifting mechanism 86 to the support surface 14. The gas source 114 is
coupled to the yoke member 92 of the lifting mechanism via conduit 115.
The yoke member 92 is formed with a channel network 116, shown
particularly in FIG. 9, for distributing the gaseous substance between the
three lifting pins 88. The gaseous substance enters through a gas inlet
118 and travels through the channel network 116 along the legs of the yoke
member 92 to socket 102. The socket 102 has a hole 120 formed therein
which extends inwardly to the interior cavity 122 of the socket. The shaft
100 fits loosely within the socket cavity 122, providing a passageway for
the gaseous substance between the shaft 100 and the socket 102. The
gaseous substance flows upwardly around the shaft 100 into the interior of
the bellows assembly 104. From the bellows assembly 104, the gas flows
between the collar 96 and the bearing 98 and upwardly through the aperture
90 formed in the electrode assembly 20 to the support surface 14. The
apertures 90 extending through the electrode assembly 20 are coated with a
dielectric material to isolate the DC voltage from the gaseous substance.
Preventing exposure of the gaseous substance to the DC voltage is of
particular importance to prevent arcing.
Transporting the gaseous substance through the lifting assembly 86 is of
particular advantage in that the number of holes formed in the electrodes
is minimized. This is particularly desirable in that it improves the
reliability of the electrode assembly 20 by minimizing areas of possible
arcing when the support system 10 is used in a plasma-enhanced processing
system. Minimizing the number of holes also reduces the manufacturing
costs of the support body 12. The apertures 90 are particularly suitable
for uniformly delivering gas to the support surface 14 because of the
symmetrical arrangement of lifting pins 88 and apertures 90.
The cooling system of the present invention includes means for preventing
catastrophic separation of the wafer 6 from the support body 12 during
processing. As is shown particularly in FIG. 10, the outer diameter of the
collar 96 of the lifting pin 88 is substantially equal to the inner
diameter of the longitudinal bore of the bearing 98. A
longitudinally-extending shallow groove 126 formed in the interior wall of
the bearing 98 and a longitudinally-extending shallow groove 128 on the
exterior of the collar 96 provide a conduit for the gaseous substance
between the bellows assembly 104 and the apertures 90. Although the
grooves 126 and 128 are substantially aligned in the present embodiment,
the grooves 126 and 128 may also be circumferentially spaced relative to
one another if desired. The shallow grooves 126 and 128 provide a
restriction which significantly reduces the flow rate of the gaseous
substance. The restriction divides the gas flow path into a first plenum
between the gas source 114 and the bearing 98 and a second plenum between
the bearing and the support surface, with the second plenum containing
only a fraction of the available gas to reducing the amount of available
gas proximate the support surface 14.
In some instances, the electrostatic charges between the wafer and the
support surface may be partially disrupted during processing. In the event
a portion of the wafer becomes separated from the support surface, the gas
in the second plenum will bleed through the resulting gap into the
processing chamber. If the entire gas supply was exposed to the support
surface, the pressure would cause the wafer to be forced from the support
surface 14. This catastrophic separation of the wafer from the support
surface is prevented because of the limited volume of gas in the second
plenum. After the gaseous substance in the second plenum has escaped, the
wafer is free to fall back against the support surface and become
reattached to the support body. With the restriction provided by the
shallow grooves 126 and 128, possible damage to the wafer because of
partial loss of the electrostatic charge is substantially eliminated.
In the present embodiment, the restriction is provided by the shallow
groove 126 formed in the interior of the bearing 98 and the shallow groove
128 formed on the exterior of the collar 96. However, it is to be
understood that the position of the restriction relative to the support
surface may be varied if desired. Moreover, other means may be used to
restrict the flow of gas between the gas source 114 and the support
surface 14 within the scope of the present invention. Because of the
restriction, additional time is required for the second plenum and the
space between the wafer and the support surface 14 to fill with gas. As a
result, the wafer may quickly reach the desired processing temperature.
Another advantage of the restriction is that it provides a capillary feed
which prevents arcing in the event you have a high potential difference
between the support surface 14 and the lifting mechanism 86.
Although not shown, with the support body of the present invention, the
gaseous substance used for cooling the wafer during processing may also be
used to raise and lower the lifting pins 88. In the modified embodiment,
the lifting pins 88 are carried by a suitable support structure and
coupled to a spring. The pressure of the gaseous substance and the springs
control movement of the Lifting pins between the extended and retracted
positions shown in FIGS. 4 and 8.
The support surface 14 of the present invention includes means for
uniformly distributing the gaseous substance across the entire wafer. As
is shown particularly in FIG. 3, a plurality of gas distribution grooves
136 are formed in the support surface 14. In the present embodiment, the
location of the apertures 90 coincide with the grooves so that the gas is
fed directly into the gas distribution grooves. The gas then flows through
the grooves and into the spaces between the rear surface of the wafer and
the support surface 14. The grooves 136 preferably divide the support
surface 14 into a plurality of segments 138, with the total surface area
of each of the segments being substantially equal. In the present
embodiment, the groove configuration includes six radially extending
grooves positioned at sixty degree intervals around the circumference of
the wafer and five circumferentially extending grooves. When the support
body 12 is used to retain an eight inch wafer, the segments 138 each have
a surface area of about 1.5 to 2.5 square inches, for example two square
inches. However, it will be understood that a greater or lesser segment
surface area is within the scope of the present invention. The
configuration of the gas distribution grooves 136 provides for a
substantially uniform distribution of gas across the entire support
surface so that the wafer may be subjected to uniform cooling throughout
the process.
The configuration of the grooves is of particular importance to avoid
ignition of the gaseous substance due to the existence of potential
between the wafer and the electrode especially in plasma-enhanced systems.
The width of the groove measures approximately 0.031 to 0.125 inches,
preferably about 0.062 inches, while the maximum depth is approximately
0.010 to 0.031 inches, preferably about 0.015 inches. As is shown
particularly in FIG. 11, the grooves 136 are curved. The edges of the
grooves have a radius of curvature R1 in the range of 0.031 to 0.062
inches while the radius of curvature R2 of the grooves is in the range of
0.031 to 0.093 inches. In the present embodiment, the outer groove edges
preferably have a radius of curvature of about 0.045 inches while the
radius of curvature of the groove is about 0.062 inches. With these
dimensions, ignition of the helium employed in the present embodiment is
substantially avoided.
With the support system 10 of the present invention, arm members 16 mount
the support body 12 to the processing chamber with the support body spaced
from the bottom of chamber as is shown for example in FIG. 1A. Removing
the support body 12 from the bottom of the chamber offers increased
flexibility in the design of the overall processing system. For example,
the pump (not shown) may be axially aligned with the support body 12,
minimizing the footprint of the overall system and improving the
effectiveness of the pump during processing. In the present embodiment,
support system 10 includes two arms 16A and 16B extending toward one wall
of the processing chamber 8. However, it is to be understood that the
number of arms may be increased or, if desired, only one arm member may be
used. Using a plurality of arm members offers several advantages,
including increased stability and reduced size. The reduced diameter of
the arm members 16 reduces the amount of interference with the symmetry of
the flow of gases to the pump. This is particularly important for pressure
sensitive applications. In the present embodiment, arm members 16A and 16B
are mounted to one wall of the chamber 8 as is shown in FIG. 2. However,
it is to be understood that the arm members may also extend toward the
corners of the processing chamber or to different chamber walls if
desired.
Arm members 16 are each formed with a longitudinally extending bore 144. As
is schematically illustrated in FIG. 2, the bore of one of the arm members
16A provides a conduit from the support body 12 for the electrical
connectors 40 and 44 coupling the electrodes 22 and 24 to the voltage
source and the electrical connector 52 coupling the RF source 54 to the
electrodes. The gas source 114, the fluid source 36 for the electrodes
assembly 20 are connected to the support body 12 through conduits 115 and
35, respectively, which extend through the bore 144 of the other arm
member 16B. Although not shown, the pneumatic lines for operating the
actuator 94 also extend through the bore 144 of the arm member 16B. By
using two arm members 16, the electrical components are safely separated
from the liquid cooling fluid as well as the gaseous substance used to
cool the wafer.
As is shown in FIGS. 1 and 1A, in the present embodiment arm members 16A,
16B are preferably mounted to a support plate 150 of carriage structure
17. The support system 10 is installed by inserting the support body 12
and arm members 16 through an opening formed in the chamber wall and
securing the support plate 150 to the exterior of the chamber with
suitable fasteners. The support body 12 may be conveniently removed by
disengaging the support plate 150 from the chamber exterior and pulling
the entire unit from the chamber. A housing 152 mounted to the opposite
side of the support plate 150 encloses components of the support system 10
such as RF/DC combiner, match network and DC supply. Coupling members (not
shown) couple the fluid source 36 for cooling the electrode assembly 20
and the gas source 114 for cooling the wafer 6 to the conduits 35 and 115,
respectively. A track structure 154 supports the support plate 150 and
housing 152, facilitating movement of the support plate 150 for insertion
of the support body 12 into or removal of the support body from the
chamber.
With carriage assembly 17, support body 12 may be efficiently and
conveniently positioned in and removed from the processing chamber. The
carriage assembly 17 improves the accessibility of the chamber for
maintenance and clean-up of a fractured wafer. Manufacture of the
processing chamber is simplified as the arm members 16 may be mounted to
the carriage structure and the necessary connections completed outside of
the processing chamber. Although use of the carriage assembly 17 is
preferred, it is to be understood that the assembly may be omitted and the
arm members 16 mounted directly to the chamber wall.
The foregoing descriptions of specific embodiments of the present invention
have been presented for purposes of illustration and description. They are
not intended to be exhaustive or to limit the invention to the precise
forms disclosed, and obviously many modifications and variations are
possible in light of the above teaching. The embodiments were chosen and
described in order to best explain the principles of the invention and its
practical application, to thereby enable others skilled in the art to best
use the invention and various embodiments with various modifications as
are suited to the particular use contemplated. It is intended that the
scope of the invention be defined by the claims appended hereto and their
equivalents.
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